Exposure age dating in glacial environments

This article is edited and drawn from:

Davies, B.J., 2022. Dating Glacial Landforms II: Radiometric Techniques, in: Haritashya, U. (Ed.), Treatise in Geomorphology (Second edition). Cryospheric Geomorphology. Elsevier, pp. 249-280. (link)

Constructing deglacial chronologies

Exposure-age dating is particularly useful for reconstructing deglacial chronologies because temperate, erosive glaciers create fresh rock surfaces, theoretically removing rock with previous exposure to the cosmic ray flux (Balco, 2011). It can be used in the absence of organic matter, and so is applicable even in dry, windy, cold environments like Antarctica, as well as temperate tropical mountain glaciers.

Cosmogenic nuclide exposure-age dating does not suffer from the limitations of methods like lichenometry or dendrochronology, which can have poorly known ecesis times and be susceptible to variations in microclimate.

Direct dating of landforms

Unlike radiocarbon dating, where there is a lag for the onset of organic matter formation and sedimentation following deglaciation, exposure-age dating provides a direct age for the date of moraine formation or glacial retreat, rather than bracketing ages (Balco, 2011).

Well-constructed and empirically validated production rates and scaling schemes mean precision is increasing and the technique widely applicable. It works over a large timescale and is relatively straightforward to apply in the field. A number of inventive methods have been applied to reconstruct deglacial chronologies (Figure 4).

Figure 4. Cartoon illustrating some of the different ways in which exposure-age dating can be used to understand deglacial histories. From Davies, 2021.

Dating glacially transported boulders

Exposure-age dating is frequently applied to date the timing of the deposition of glacially transported boulders (Applegate et al., 2012; Davies et al., 2019; Hein et al., 2010; Heyman et al., 2011; Mendelová et al., 2020; Putnam et al., 2013). 10Be exposure-age dating of felsic, phaneritic or sandstone glacially transported boulders on glacial moraines is perhaps the most widely applied form of exposure age dating (e.g., Davies et al., 2018; Joy et al., 2014; Kaplan et al., 2020, 2016; Kelly et al., 2008; Reusche et al., 2014; Reynhout et al., 2019; Sagredo et al., 2018).

In this scenario, boulders in a stable position on a moraine ridge crest are sampled, and the age is presumed to be equal to the age of moraine formation (Figure 4). This application of the method is particularly useful because moraines represent a decisive period of time when the glacier was in equilibrium with climate, and maintained its position long enough to build the moraine.

The timing of moraine formation can be compared to proxy records of palaeoclimate, shedding insights into glacier-climate interactions. Therefore, reconstructing the timing of moraine formation is of particular interest to glacial geologists (Kaplan et al., 2020; Mackintosh et al., 2017; Sagredo et al., 2018).

Using a notebook for scale beside this boulder. A glacially transported boulder.

Other studies have focused on using exposure ages to reconstruct vertical thinning histories, rather than horizontal recession from a moraine (e.g., Boex et al., 2013; Hormes et al., 2013; Johnson et al., 2014, 2012; Lindow et al., 2014; Mackintosh et al., 2007; Stone et al., 2003). Here, samples from either ice-scoured bedrock or glacially transported boulders or cobbles are taken in a vertical transect from a mountain summit downwards. Linear rates of deglaciation can be estimated by calculating the distance and age offset between dated positions (Jones et al., 2019). These data can be used to calculate rates of ice surface lowering (Small et al., 2019).

Ice-scoured bedrock

Exposure age dating can be used to constrain the timing of ice recession from ice-scoured bedrock, and can be paired with dating glacially transported boulders (Corbett et al., 2013, 2011). This can avoid the limitations associated with moraine degradation (Ivy-Ochs and Briner, 2014). However, glacial erosion of less than ~3 m can lead to incomplete removal of nuclides that accumulated during previous exposures.

This method is likely to be therefore more appropriate in temperate, deeply eroded alpine valleys (e.g., Guido et al., 2007), with inheritance more likely in high elevation, high latitude regions dominated by cold-based ice (Fabel et al., 2002; Stroeven et al., 2002; Sugden et al., 2005).

Cobbles and pebbles

In places devoid of sediment, soil, vegetation and organic matter that can obscure the cosmogenic nuclide signal, and where large boulders are few in number, such as nunataks above ice sheets, 10Be exposure ages can be obtained from smaller glacially abraded quartz-bearing cobbles and pebbles on bedrock (Balco et al., 2013; Bentley et al., 2010; Dong et al., 2016; Hein et al., 2017).

Dating erratic cobbles or boulders directly on bedrock can be especially useful if inheritance (nuclides from a previous exposure) is anticipated within the bedrock due to low erosion rates under cold-based ice (Balco, 2011; Bentley et al., 2010; Fabel et al., 2012). Sampling cobbles or boulders resting on bedrock limits clast cycling through the active layer (Hein et al., 2016). If signs of glacial abrasion and transport (erratic lithology, striations, edge-rounded) are clear on the cobbles, this may be taken to indicate negligible erosion.

Self-shielding and post-depositional movement is limited in clasts resting on flat bedrock and thinner than ~5 cm (Hein et al., 2016). Care should be taken to ensure that samples could not have reached these positions by colluvial processes after deglaciation (Balco et al., 2013).

For older glaciations where moraine degradation and boulder erosion can make cosmogenic nuclide exposure age dating challenging, cobbles on outwash gravels can also be targeted (Hein et al., 2009). In Patagonia, the arid, windy conditions help ensure continuous surface exposure of surface cobbles. Here, these data revealed a glacial advance during MIS 8, with outwash cobbles yielding exposure ages ~100 ka older than moraine boulders.

Dating lake drainage

Exposure ages can also be used to date the timing of ice-dammed glacial lake drainage (Davies et al., 2018; Fabel et al., 2010; Hein et al., 2010; Thorndycraft et al., 2019) and for dating cobbles within glaciofluvial outwash (Hein et al., 2017, 2011). This is applicable because the cosmic ray flux is strongly attenuated by 2 m of water (Gosse and Phillips, 2001), so boulders deposited below shorelines or perched deltas will constrain the timing of lake-level fall. Figure 4 illustrates some of the ways in which exposure-age dating can be used to understand deglacial chronologies.

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